I am often asked “how can doctors remember the different antibiotic spectrums of activity”? This is a difficult task as there are so many. Personally I have never found lists of medical information that helpful, I would much rather come up with a mnemonic. However, I’m stumped for a mnemonic to remember antibiotic spectrum of activity, so a list it must be. What I have done though is reduce a very long list into six short related lists, which I hope are easier to remember.

NOT knowing the spectrum of activity of the antibiotic they are prescribing is high up there on the list of situations when doctors make a mistake. NOT knowing leads to patients being given antibiotics which have no activity against the bacteria which are known or suspected to be the cause of an infection. My current pet hate is the use of Teicoplanin to treat UTIs (see Teicoplanin blog...) but I also come across patients being treated for Pseudomonas spp. with Co-amoxiclav which has no activity against Pseudomonas spp., or an anaerobic infection being treated with both Co-amoxiclav AND Metronidazole when either alone would be enough. Sometimes it seems doctors are just guessing or picking their favourites!

Remembering the 6 lists below will help to ensure patients are on an antibiotic that is active against their infection.

Example A: A patient has a perforated appendix. Which antibiotics could be used to cover the mixed bowel flora that will have leaked into the patient’s peritoneum? Any of Co-amoxiclav, Piptazobactam, Ertapenem, Imipenem or Meropenem could be used alone as they are broad spectrum AND cover anaerobes. If Cefuroxime, Ceftriaxone or Cefotaxime were used then Metronidazole would need to be added to cover the anaerobes. You would not use Cefuroxime, Ceftriaxone or Cefotaxime with Clindamycin as this poses too high a risk for Clostridium difficile associated diarrhoea (CDAD).Example B: In patients with serious penicillin allergies Beta-lactams including Co-amoxiclav, Piptazobactam, Ertapenem, Imipenem or Meropenem are contraindicated. Combinations of antibiotics are therefore needed to cover the gut flora, such as Teicoplanin OR Vancomycin (which only have Gram-positive activity) PLUS Metronidazole (which only covers anaerobes) PLUS either Ciprofloxacin OR Gentamicin (both of which have excellent Gran-negative activity).List 3

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Example C: A Cystic Fibrosis (CF) patient has an exacerbation of their CF caused by Pseudomonas aeruginosa. What antibiotics would they be treated with? They would be treated with a combination of antibiotics active against Pseudomonas aeruginosa such as IV Meropenem PLUS IV Tobramycin PLUS nebulised Colistin. Why a combination of antimicrobials? Combination antimicrobials are used to try and prevent further development of resistance (Pseudomonas aeruginosa is notorious for acquiring new resistance mechanisms), in non-CF patients two antimicrobials are usually enough. It is unlikely that bacteria will become resistant to two or more antimicrobials at the same time therefore resistance is less likely to occur. Even if the bacteria do become resistant to one of the antimicrobials the other agent(s) will be able to kill the bacteria.

List 4 and 5

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Example D: A patient has a skin and soft tissue infection but also has a severe allergy to penicillins. Which antibiotics could be used to treat them? Skin and soft tissue infections are almost always caused by Gram-positive bacteria such as Staphylococcus aureus or Beta-haemolytic streptococci therefore Vancomycin, Teicoplanin, Linezolid and Daptomycin would all be capable of treating the infection. It may sound obvious but you cannot use an antimicrobial only active against Gram-negatives to treat a Gram-positive, therefore Ceftazidime, Colistin or Aztreonam would definitely not be appropriate as they do not have any activity against the Gram-positive causes. Broad-spectrum antibiotics would treat the infection but it would be unnecessary as the bacteria are known to be Gram-positive. Broad spectrum antibiotics in this case might lead to unwanted side effects such as CDAD.

List 6

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Example E: A patient has pneumonia caused by Mycoplasma pneumoniae (a non-culturable bacterium). What antibiotic should they be treated with? Erythromycin, Clarithromycin, Azithromycin, Ciprofloxacin, Levofloxacin and Doxycycline all could be used to treat this patient.

If you can learn these six lists it starts to become much easier to understand why certain antibiotics are chosen to treat certain infections. It also enables you to choose alternatives when patients are allergic to first line antibiotics found in guidelines. Download a handy postcard of the 6 lists and keep it in your book Microbiology Nuts and Bolts.

Give it a try and see if it helps. Pass it on too. If you can think of any easy way of remembering them then let me know, I’m always keen to know how others have learned their microbiology.